SEVERAL TESTABLE HYPOTHESES FOR INFERRING BIOGEOCHEMICAL CONDITIONS IN PALEOLAKE WATERS BASED ON THE CHEMISTRY OF MG-SILICATE CLAYS

Alkaline (pH ≳ 8.5) lakes are amongst the most biologically productive environments on Earth. Aluminum-poor magnesian clays (e.g., sepiolite, stevensite, and kerolite) are common constituents of the sediments they deposit, and the mineralogy of these clays may provide a useful record of the biogeochemistry of the lake waters from which they were precipitated. Here, we integrate recently published and new experimental data; empirical, thermodynamic, and kinetic relationships developed from these data; and a compilation of lake water chemistry to develop several testable hypotheses for inferring the biogeochemical conditions in paleolake waters based on the chemistry of Mg-silicate clays. These hypotheses are as follows: (1) Mineralogy and chemistry of authigenic (i.e., Al-poor) Mg-silicate clays in the sedimentary record of alkaline lakes directly reflect the mineralogy and chemistry of the initially precipitated material, even if that material was amorphous at the time of deposition; (2) Ratios of the concentrations of Mg and Si in the paleolake waters can be estimated from observed Mg-silicate clay mineralogy using an empirical calibration based on laboratory experiments; (3) At pH ≳ 9.5, diatoms are rarely the dominant primary producer in alkaline lakes, are thus not silica-limiting, and are consequently not responsible for significant undersaturation of silica with respect to amorphous silica; (4) Fluid Mg/Si ratios rising above a critical baseline, based on the critical supersaturation of Mg-silicate clays and equilibrium with respect to amorphous silica, in the absence of diatom activity are most likely caused by the precipitation of silicate clays; (5) Fluid Mg/Si ratios falling below this critical baseline in the absence of diatom activity are most likely caused by the precipitation of magnesium carbonates; and (6) Amorphous silica precipitation with Mg-silicate clays indicates fluid pH below a threshold. The focused testing of these hypotheses against modern occurrences and the geologic record of alkaline lakes can have profound implications for the interpretation of the paleo-biogeochemistry and paleohabitability of these systems on Earth and beyond.